In recent years, image projection devices such as liquid crystal projectors and DMD projectors are broadly used as systems for realizing a large-screen picture. In general, these kinds of image projection devices broadly employ high-pressure discharge lamps each exhibiting a high intensity. The image projection device need focus light on an extremely small area such as a liquid crystal panel, and therefore the high-pressure discharge lamp is required to serve as an approximate point light source as well as have a high intensity. Such being the case, attention is being given as a promising light source to a short-arc-type super-high pressure mercury lamp that most approximates a point light source among high-pressure discharge lamps and that has the advantage of high intensity.
A known short arc super-high pressure mercury lamp 1000 will be described with reference to FIG. 1.
A lamp 1000 includes a generally spherical luminous bulb (bulb) 100 composed of silica glass, and a pair of sealing parts 101a and 101b similarly composed of silica glass and coupled to the luminous bulb 100. The inside of the luminous bulb 100 includes a discharge space, in which mercury (the amount of encapsulated mercury: 150 through 250 mg/cm3 relative to the internal volume of the luminous bulb, for example) that is a luminous material, a rare gas (a-few-tens-of-kPa argon, for example) and a small amount of halogen are encapsulated.
In the discharge space, a pair of tungsten electrodes (W electrodes) 102 are arranged so as to be opposed to each other at a given interval, and a coil (not shown) may be wound around each of the tips of the electrodes 102. The W electrodes 102 are welded to molybdenum foils (Mo foils) 103 in the sealing parts 101a and 101b, respectively, so that the W electrodes 102 and the Mo foils 103 are electrically connected to each other.
Each of the sealing parts 101a and 101b includes a glass part 105 extending from the luminous bulb 100 and a Mo foil 103. The glass part 105 and the Mo foil 103 are crimped, thereby holding the hermeticity of the discharge space in the luminous bulb 100. Both of the glass part 105 and the Mo foil 103 cannot be integrated together because both of them have different coefficients of thermal expansion. However, the plastic deformation of the Mo foil 103 enables a gap produced between the Mo foil 103 and the glass part 105 to be filled in. That is, the sealing parts 101a and 101b seal the inside of the luminous bulb 100 using a so-called foil sealing technology.
Each of the Mo foils 103 includes an outer lead 104 composed of molybdenum at the side opposite to each of the W electrodes 102. The Mo foil 103 and the outer lead 104 are welded to each other so that both of them are electrically connected to each other. The outer lead 104 is to be electrically connected to a member arranged around the lamp 1000 (not shown).
Next, the operating principles of the lamp 1000 will be described briefly. When a starting voltage is applied via the outer leads 104 and the Mo foils 103 to the W electrodes 102, argon (Ar) discharges to increase the temperature in the discharge space of the luminous bulb 100, so that mercury is heated and vaporized. Thereafter, mercury atoms are excited in the middle of an arc between the W electrodes 102 to emit light. The higher the mercury vapor pressure of the lamp 1000, the more obtained the light output can be. Therefore, a lamp having a higher mercury vapor pressure is more suitable for a light source of the image projection device. However, the lamp 1000 is used under a mercury vapor pressure of 15 through 25 MPa in terms of the physical strength of the luminous bulb 100 against pressure.
With the widespread use of image projection devices, there has been an increasing demand, to high-pressure discharge lamps (more particularly, super-high pressure mercury lamps) as light sources for image projection devices, for excellent properties, and the development of high-pressure discharge lamps have actively been carried out so as to meet the demand.
Under these circumstances, a high-pressure discharge lamp was developed in which cavities containing a rare gas and a mercury vapor were provided in sealing parts to allow the lamp to be started at low voltage. This lamp is disclosed in International Publication WO00/77826. FIGS. 2, 3(a) and 3(b) show the structure of this lamp. FIGS. 3(a) and 3(b) are a plan view and a side view of the structure of the lamp shown in FIG. 2, respectively.
A lamp 2000 shown in FIG. 2 is provided with cavities 150 in the sealing parts 101a and 101b, and an antenna 120 is arranged around the cavity 150 of the sealing part 101b. The antenna 120 is connected through a lead 121 to an outer lead of the sealing part 101a. A second antenna is arranged around a neck part between the sealing part 101b and the luminous bulb 100. Coils 112 are wound around the tips of the W electrodes 102. Here, the same numerals as in FIG. 1 denote the same members, and the descriptions thereof are not given.
The lamp 2000 allows discharge to occur between a metal foil 103 located in the cavity 150 in which gas is encapsulated and the antenna 120, thereby achieving its start at low voltage. The same international publication discloses that at the start of the lamp from a state where the lamp is cold (cold start), the lamp can start at a voltage of 1 kV.
However, as shown in FIGS. 3(a) and 3(b), the lamp 2000 has edges of the Mo foil 103 exposed to the inside of the cavity 150. Thus, there occurs a new problem that the discharge produced in the cavity 150 causes the deterioration of the Mo foil 103. When the use of the lamp causes the Mo foil 103 to be deteriorated, this results in a short lamp life. The reason is that the lamp 2000 including the sealing parts 101a and 101b holds the hermeticity of the inside of the luminous bulb 100 by foil sealing, as described above. That is, the lamp 2000 including the cavities 150 in the sealing parts, unlike the lamp 1000 shown in FIG. 1 and including no cavity 150, is less likely to become a practical high-pressure discharge lamp unless it is given the function of starting at low voltage as well as its life can be prevented from being shortened.
The present invention is made in view of the above-described problems, and it is a main object thereof to provide a discharge lamp that can start at low voltage and also suppress the deterioration of foils to prevent its life from being shortened.